This application claims benefit of Japanese application number 2000-182615 filed Jun. 19, 2000.
The present invention relates to a process for copolymerizing tetrafluoroethylene (TFE) and perfluoro(alkyl vinyl ether) (PAVE) to give a melt-fabricable copolymer with uniform distribution of the monomers in the polymer. It further relates to a process for manufacture which enables the formation of a melt-fabricable copolymer of tetrafluoroethylene and perfluoro(alkyl vinyl ether) having narrow molecular weight distribution and having excellent mechanical properties such as flex life.
TFE/PAVE copolymer (PFA), has the advantage over TFE homopolymer (PTFE) that is melt-fabricable, and yet retains the excellent properties of PTFE such as heat resistance, corrosion resistance, weathering resistance, and the like. By melt-fabricable is meant that the copolymer can be processed into shaped articles such as films, fibers, tubes, wire coatings and the like by conventional melt extruding means. Because of this, PFA finds extensive commercial use.
In PFA it is desirable that the monomer be incorporated uniformly in the polymer chain, as determined by the concentrations and relative reactivities of the monomers toward polymerization. However, because TFE is a more reactive monomer than PAVE, and has a strong tendency to homopolymerize, attempts to copolymerize TFE with PAVE can yield polymer in which the monomers are not incorporated uniformly, but rather with TFE-rich portions and PAVE rich portions, and possibly even some PTFE. The portions of the polymer richer in TFE are higher melting, and PTFE may not melt at all. This can affect the melt-processibility of the polymer, and cause xe2x80x9cgelxe2x80x9d, bits of unmelted or imperfectly melted polymer, which is especially noticeable in film extrusion or molding.
Nonuniform incorporation of monomer can broaden molecular weight distribution (MWD), with excessive TFE incorporation giving high molecular weight polymer. Because PFA grades are classified by melt viscosity, polymer with too much high molecular weight component will also require excessive low molecular weight component to be in specification with regard to melt viscosity, usually expressed as melt flow rate (MFR). The result is an overbroad molecular weight distribution. The presence of greater amounts of low molecular weight material can affect the durability of articles molded therefrom, for example by reducing flex life. Therefore, as has been disclosed in U.S. Pat. No. 3,635,926, PFA with a broad molecular weight distribution tends to give inferior physical properties compared to PFA with a narrower molecular weight distribution even at equivalent melt viscosities.
Flex life, an indication of strength against repeated flexing, as a function of MFR, PAVE content, and MWD, can be measured by performance testing, such as in accordance with the xe2x80x9cMIT Flex Testxe2x80x9d, ASTM D 2176, or can be expressed by the equation given below, where the flex life (number of cycles to failure) is [FL]; PAVE content (% by weight) is [PAVE]; MFR (g/10 min) is [MFR]; and molecular weight distribution is represented by the molecular weight distribution index [MWDI], which is defined in the Example Section (xe2x80x9clnxe2x80x9d is the natural logarithm):
ln[FL]=B1+B2xc2x7ln[MFR]+B3xc2x7ln[MWDI]+B4xc2x7ln[PAVE]
The equation suggests a high flex life PFA can be obtained by reducing MFR, increasing the PAVE content, and/or increasing MWDI. For example, when coefficients B1-B4 are calculated from observed values from TFE/PPVE copolymer, equation (1) below results.
ln[FL]=11.208xe2x88x921.695xc2x7ln[MFR]xe2x88x927.846xc2x7ln[MWDI]+3.648xc2x7ln[PAVE]xe2x80x83xe2x80x83(1) 
With TFE/PAVE polymers of other perfluoro(alkyl vinyl ethers), the coefficients are slightly different, but the conclusion is the same. In actual flex life testing, flex life improves as molecular weight distribution narrows (uniformity increases), copolymer composition and melt viscosity being held constant.
MFR must be high enough to permit melt processing of the PFA; the PAVE content must also be limited within a certain narrow range in view of PFA physical properties and for economic reasons. Therefore, with the MFR and PAVE content fixed, MWDI presents an important means for improving the physical properties. For example, if PAVE content is fixed at 5.5% by weight and MFR at 5.0 g/10 minutes, the flex life as a function of MWDI is represented by Equation 2 below, which shows that a small variation of MWDI will significantly affect the flex life.
ln[FL]=14.70xe2x88x927.846xc2x7ln[MWDI]xe2x80x83xe2x80x83(2) 
The above reasoning leads to the conclusion that one should make a copolymer with as uniform a distribution as possible with respect to TFE and PAVE and a narrow molecular weight distribution by suppressing the homopolymerization of the TFE monomer in the manufacture of PFA.
To increase copolymer uniformity, it has been traditional to polymerize in a chlorofluorocarbon (CFC) solvent such as CFC-113 (CFCl2xe2x80x94CF2Cl) or CFC-114 (CF2Clxe2x80x94CF2Cl). However, the use of CFCs has been restricted for environmental reasons. Aqueous emulsion polymerization is an alternative method, but it is more difficult to obtain uniform copolymer in aqueous polymerization. U.S. Pat. No. 3,635,926 discloses the use of gaseous chain transfer agents such as methane, ethane, and hydrogen as a way to narrow molecular weight distribution in TFE/PAVE copolymers. Improvements are needed in aqueous polymerization of TFE/PAVE copolymers to further narrow molecular weight distribution.
As a result of studies by the inventors to overcome the above problems and to polymerize so as to generate a more uniform copolymer with a narrower molecular weight distribution for improved flex life, they have discovered that copolymerization of TFE and PAVE in the presence of a terpene in an aqueous polymerization medium produces a melt-fabricable TFE/PAVE copolymer (PFA) having a uniformly distributed PAVE. The small amount of terpene added to the polymerization system does not decrease the rate of polymerization, but is present in an amount that is effective to improve the uniformity of the resin by narrowing the molecular weight distribution.
The present invention is a process for manufacturing PFA, which comprises copolymerizing TFE and PAVE in the presence of a terpene in an aqueous polymerization medium. The resulting PFA copolymer has a narrower molecular weight distribution than obtained heretofore, this narrower molecular weight distribution being characterized by a half-width value in its differential scanning calorimeter (DSC) melting peak which is at least 10% less than the half width value of the copolymer when made without the presence of the terpene.
The comonomer PAVE (perfluoro(alkyl vinyl ether)) used in this invention is a compound that is also called perfluoroalkoxytrifluoroethylene, represented by formula 3 below.
CF2xe2x95x90CFxe2x80x94Oxe2x80x94CnF(2n+1)xe2x80x83xe2x80x83(3) 
In the PAVE of this invention n=1-10, preferably n=1-3, exemplified by such PAVEs as perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), perfluoro(propyl vinyl ether) (PPVE), more preferably by perfluoro(propyl vinyl ether) (PPVE).
The PAVE content in the PFA of this invention is sufficient to make the copolymer melt-fabricable and is about 0.5-20 mole %, preferably about 1 to 15 mole %, more preferably about 2 to 10 mole %.
Copolymerization of TFE with PAVE is carried out in an aqueous medium. The polymerization medium is essentially deionized water, optionally containing a small amount of other solvents but not more than about 5% by volume of the total volume of solvent.
To control the molecular weight and molecular weight distribution, the aqueous emulsion polymerization process is carried out, preferably using a gaseous chain transfer agent such as hydrogen, methane, or ethane as disclosed in U.S. Pat. No. 3,635,926. According to the patent, gaseous chain transfer agent is also effective for narrowing the molecular weight distribution, resulting in improved flex life, but the addition of such a chain transfer agent alone is inadequate in the aqueous polymerization process of this invention for the formation of a sufficiently narrow molecular weight distribution polymer. It has been discovered that addition of terpene has the effect of producing a uniform copolymer with a narrow molecular weight distribution in the copolymerization of TFE and PAVE in an aqueous polymerization medium of this invention. A commercially excellent PFA with narrow molecular weight distribution is obtained when a terpene is added.
In the case of solution polymerization using a chlorofluorocarbon or hydrofluorocarbon or a suspension polymerization in a mixed solvent with a large amount of nonaqueous solvent, there is no beneficial effect of adding a terpene.
The polymerization initiator used is a conventional organic peroxide polymerization initiator, a redox polymerization initiator or the like, such as bis(fluoroacyl)peroxide, bis(chlorofluoroacyl)-peroxide, a diacyl peroxide, a dialkyl peroxy dicarbonate, a peroxy diester, or a persulfate salt.
The surfactant used for the emulsion polymerization may be a conventional surfactant, preferably ammonium perfluorooctanoate (C-8), which is inert to chain transfer.
The terpene added to the polymerization system is preferably one expressed by the molecular formula represented by general formula (4) below.
(C5H8)n (n=1-3)xe2x80x83xe2x80x83(4) 
A typical and readily available terpene is limonene (C10H16).
The amount of terpene added should be about 1-100 ppm, preferably about 1-20 ppm, with respect to the total weight of monomers, TFE and PAVE, in the polymerization kettle. Lower concentrations of terpene are less effective in regulating the molecular weight distribution, while too high a concentration can cause polymer discoloration.
Any appropriate method may be selected as a way to add the terpene; for example, it may be mixed with the TFE or comonomer PAVE, which is charged to the polymerization kettle; the terpene may be directly charged to the polymerization kettle; or the terpene may be dissolved in the surfactant solution, which is then charged to the polymerization kettle.
The terpene of this invention is not a substitute for gas phase chain transfer agent. The latter, in addition to acting as a chain transfer agent in the emulsion phase, also reduces polymerization in the gas phase, which is predominately composed of TFE monomer, and to retard formation of high molecular weight PTFE in the gas phase. The terpene of this invention will beneficially affect molecular weight distribution independent of the gas phase chain transfer agent. However, it will generally be desirable to use gas phase chain transfer agent in addition to the terpene.
The melt-fabricable copolymers of TFE and PAVE obtained in this invention are characterized by flow at a temperature above their melting points, which will differ depending upon the PAVE type, its content, the molecular weight, and the like. The copolymers preferably have a melt flow rate (MFR) at 372xc2x0 C. of about 0.5-500 g/10 min, preferably about 0.5-100 g/10 min, more preferably about 1 to 50 g/10 min, and most preferably about 1 to 40 g/10 min.
Copolymerization in this invention in the presence of a terpene gives a product with a narrow melting range, which is defined by a sharp peak in a melt curve in a DSC measurement, with a half-width value of preferably not more than 8xc2x0 C. in the melting peak in the DSC at a heating rate of 10xc2x0 C./min. This small half-width value indicates relatively uniform segments of xcx9cCF2xe2x80x94CF2xcx9c repeat units, denoting the uniform distribution of the PAVE units. For comparison, commercial grades of TFE/PPVE polymer of similar composition and made by a similar process without added terpene have greater half-width values: Teflon((copyright) PFA 350, 13.4xc2x0 C.; Teflon(copyright) PFA 440, 11.7xc2x0 C.